Fuel cell UPS have also been developed in recent years using hydrogen and a fuel cell as a power source potentially providing long runtimes in a small space but at a premium price.

Rotary

Rotary uninterruptible power supply equipment uses a motor-generator system to create a perfect sine wave output. These units can be configured as (1) a motor driving a mechanically connected generator, or (2) a combined synchronous/synchronous motor/generator wound in alternating slots of the field and stator. The motor side of the unit in case #2 can be driven directly by an AC power source or by a 6-step double-conversion motor drive. Case #1 uses an integrated flywheel as a short-term energy source instead of batteries to allow time for external, electrically coupled gensets to start and be brought online. Case #2 can use batteries or a free-standing electrically coupled flywheel as the short-term energy source. Sometimes, in case #1, the flywheel itself is used to start the generator in a mechanically coupled diesel configuration.

Rotary UPS equipment is far more tolerant of lightning strikes than static equipment. It can provide up to 17x fault clearing capabilities without going to bypass. These units provide superior current inrush handling for inductive loads such as motor startup or compressor loads as well as medical MRI and cath lab equipment.

The life cycle of these units is usually far greater than that of their static siblings, up to 30 years or more but they also cost a lot more.

Standby (offline)

With this design, the UPS simply passes utility power through to the load until either a power failure, sag or spike occurs, at which point, the UPS switches the load onto battery power and disconnects the utility power until it returns to an acceptable level. In this design, the UPS unit only charges the battery when it is running on utility power. This design is the most cost effective and typically makes use of a square wave or modified square wave inverter. These units are typically found in units 600 VA and below and designed for home use. This design solves problems 1 – 3, however the disadvantage of this is that any of the power problems numbered 4 - 9 will cause the UPS to switch to battery, and may cause it to completely drain the battery and shut off even though line voltage is still present true.

Line-interactive

Line interactive UPS units are designed so that the inverter is always connected to the output of the UPS. When line power is present, the inverter operates in reverse to charge the battery. When utility power fails, the UPS reverses the power flow from the inverter and provides power to the load. This design provides better filtering than a standby unit because the inverter is always connected to the load.

Line interactive units typically will incorporate an automatic voltage regulator. AVR allows the UPS to effectively step-up or step-down the incoming line voltage without switching to battery power. This allows the UPS to correct most long term over-voltages or under-voltages without draining the batteries. Another advantage is that it reduces the number of transfers to battery which extends the lifetime of the batteries.

Line-interactive UPS units are the most common design for units in the 0.5 kVA to 5 kVA range. They are typically used in small server environments but are not as reliable as Delta or Double conversion used in larger applications.

Delta conversion online

Delta conversion is a new in concept online technology. Unlike offline technology, no "switch on" time is required. Like other True-Online technology, a continuous separation of load and primary power is offered except the frequency.  With Delta Conversion, frequency is synchronized with main input.

Delta conversion, as it's name implies, involves having the inverter generate the "difference" between the line voltage and the desired voltage. It does this by magnetically coupling the line on the primary side with the inverter on the secondary. When the line voltage is within the acceptable range, no power is drawn from the inverter and the load is directly powered by the source. This gives delta conversion extremely high efficiencies at this sweet spot. When the line voltage deviates from the acceptable range, the inverter delivers a voltage on the secondary winding of the transformer which induces a voltage across the primary which boosts or trims the source.

The battery is charged by an inverter in parallel with the load. This inverter operates in both directions powering the DC bus from the AC line or vice versa. When the source cuts out, the inverter powering the transformer turns off which turns off the source. This is because no current on the secondary winding means no current through the primary which is connected to the source. The inverter that charges the battery then operates in reverse powering the load.

The reason for this technology (which all True-Online UPS have) is to synchronize the frequency with the main input (normal operation mode). This is necessary when UPS transfers from normal mode to bypass mode. It will request a frequency synchronize between input/output for a successful transfer. In fact, other True-Online UPS will supply separation frequency only when bypass mode is disabled. With the UPS, bypass mode is very important. Like brakes or air bags in a car, it is a "fail safe"; it will help to supply continuous power in case of UPS fault, overload, etc.

Dual conversion online
(Double Conversion)

Dual conversion uninterruptible power supplies operate by converting incoming utility AC power to DC and then convert the DC back to AC power connected to the load. This is also called "double conversion" or "dual conversion." The batteries are directly connected to the DC level, which provides an excellent filter for removing line noise. Effectively, this design isolates the load from the incoming power and regenerates the sine wave. This yields many benefits. First, this design will protect against all 9 of the common power problems. It allows the UPS to use almost any incoming power, including generators. Second, this design allows the UPS to change incoming voltages and even frequencies easily. Third, because the load is always powered by the inverter, when power fails, there is no transfer time while the UPS switches from line power to battery power. While for most computer applications the switching time is not a problem, some industrial equipment can be harmed (air conditioner compressors for example), making this a better solution.

Online units are typically used in environments with sensitive equipment or environments where a generator is used to provide backup power to the UPS. Almost all UPS units 5 kVA and above are online, although they can be found in capacities as small as 1000 VA.

Because the AC output must be constantly generated by the UPS inverter, any failure of this inverter could potentially cause an interruption to the connected load. This is the very thing that the UPS is designed to avoid in the first place. As a measure of reliability, nearly all double conversion UPS units have a sophisticated monitoring system on the output that senses when the voltage or current goes out of specification. On larger UPS units, a solid state based bypass is then activated to shunt incoming AC directly to the attached load without interruption. This can be due to UPS output inverter failure, input rectifier failure (and eventual battery discharge) or other internal UPS failure. By detecting and shunting raw incoming AC directly to the load, these failures can be avoided but power filtering is elimnated or reduced. Another mode that requires bypass is fault clearing mode. A fault on the connected load, such as a short circuit in a power distribution panel or computer server power supply, may require more current than the UPS inverter can produce, in order for a fuse to blow, or a breaker to trip. During this mode of operation, bypass current is supplied directly to the output until the fault condition is resolved, usually in a matter of milliseconds. Without bypass modes, all of the other attached loads could lose power if even one experiences a fault. In addition to a high speed electronic bypass, most large (greater than 10KVA) UPS units have one or more layers of switches and/or breakers connected to the input and output to allow the entire unit to be bypassed, shut down, and isolated for maintenance without the connected loads being affected.

The ability for a UPS to bypass itself during abnormal conditions drastically increases the reliability of its output. Double online UPSes have their frequency and phase synchronized with its input (normal operation mode). It's very necessary when a UPS transfers from normal mode to bypass mode not to disturb the power in any way, including not abruptly changing phase or frequency. This can disturb timing circuits (extra zero crosses in the sine wave) or cause jerks in a motors output.

Larger UPSes are expensive but are often a better value. Fewer larger UPSes tend to be more reliable than many smaller units (that don't contain bypass circuits). These units may be marketed as Power conditioners. In data centers, multiple sets of UPS units may run in parallel providing dual sources of conditioned power to static switches that then send power to server loads. In such a system, a complete UPS failure can occur without the loads connected to the switches being affected.

Ferro-resonant

Ferro-resonant units operate in the same way as a standby UPS unit with the exception that a ferro-resonant transformer is used to filter the output. This transformer is designed to hold energy long enough to cover the time between switching from line power to battery power and effectively eliminates the transfer time. Because the transformer typically gives off a lot of heat, these units are typically large, bulky, and and are not as efficient as other technologies but are very sturdy "workhorse" type UPSes".

DC systems

Many systems used in telecommunications use DC power (often 48 V). Rather than converting AC to DC to charge batteries, then DC to AC and then convert it back to DC again, some equipment accepts 48 V DC power directly. By simply converting AC power to DC power and adding batteries to the DC side, one or more conversion steps can be saved. There has been much experimentation with DC power for computer servers, in the hope of reducing the likelihood of failure and the cost of equipment. Because there is more current to transfer the same amount of energy at the lower DC voltage, larger conductors are needed, and more energy is lost as heat. On the surface, eliminating a conversion step may seem more reliable, but the ability of online double conversion AC systems to entirely remove themselves from operation and transfer to bypass mode during certain UPS failures and maintenance allows for the connected servers to continue to function on unconditioned AC power while the UPS is repaired. DC-based power systems do not have this luxury, as it requires that all equipment has special DC power inputs that cannot utilize AC voltages in the event of a main DC rectifier or power distribution failure. DC has typically been the dominant power source for telecommunications, and AC has typically been the dominant source for computers and servers. Higher voltage DC (370 volts), however, may find an eventual use in data center applications.